1. Field of the Invention
The invention relates to a multi-band low noise amplifier capable of operating in a plurality of band modes, and more particularly, to a method for connecting an output amplifier to at least one of a plurality of input amplifiers in cascode connection at a lowest-impedance node, so as to reduce the impedance of the multi-band low noise amplifier.
2. Description of the Prior Art
Following the trend of wireless communications development, the upcoming wireless communication systems may include a variety of standards. A mobile phone may operate in a wireless communications system with different frequency bands, such as GSM900+DCS1800+PCS1900 (GSM: global system for mobile communication; DCS1800 is also called GSM1800; PCS1900 is also called GSM1900), AMPS+GSM (AMPS: advance mobile phone service), CDMA+GSM (CDMA: code division multiple access), GSM+bluetooth, and GSM+WLAN (WLAN: wireless local area network). Moreover, integrated with multi-media services, communication instruments can expand their functionality and compatibility. Therefore, for each component of the wireless communication terminal instrument, a related function for processing signals of various standards should be equipped. Regarding a low noise amplifier (LNA), the multi-band low noise amplifier, which can be operated in a plurality of band modes, becomes essential in a receiver of the wireless communication system.
The multi-band low noise amplifier is mainly used to provide required gains and sensitivity for a received signal from an antenna. Since the multi-band low noise amplifier is installed in the front end of the receiver for processing generally weak signals, performances of the multi-band low noise amplifier, such as the noise figure, the RF gain, and linearity, are very important to the overall performances of the receiver. FIG. 1 is a functional block diagram of a conventional receiver 10 of the wireless communication system. The receiver 10 is mainly applied in the wireless communication system operated in a frequency band of 0.9 GHz to 10 GHz since most of the commercial wireless communication systems, such as GSM900/DCS1800/PCS1900, blue-tooth, and WLAN, are operated in this frequency band. The receiver 10 includes a multi-band antenna 12, a set of filters 14, a multi-band low noise amplifier 16, a mixer 18, a local oscillator generator 20, and a signal processing module 22. The multi-band antenna 12 can be used to receive an RF signal RF of different frequencies, and the present embodiment includes two band modes, a high band mode and a low band mode. In the present embodiment, RF signal RF received by the multi-band antenna 12 can be classified into a high RF signal HRF and a low RF signal LRF, which respectively pass a high band filter 14H and a low band filter 14L for filtering process to respectively become a high band input signal HSI and a low band input signal LSI. The high band input signal HSI and the low band input signal LSI will be respectively amplified by the multi-band low noise amplifier 16 by a gain ratio. After the multi-band low noise amplifier 16 outputs the amplified signal, the mixer 18 can cooperate with the local oscillator generator 20 to down-convert the frequency of the outputted signal to a predetermined frequency, and the signal processing module 22 then proceeds with advanced operations of intermediate-frequency (IF) amplification, signal demodulation, and image rejection.
According to the prior art receiver 10 shown in FIG. 1, the multi-band low noise amplifier 16 is substantially a combination of two single-band low noise amplifiers, a high band low noise amplifier 16H and a low band low noise amplifier l6L, for respectively receiving and processing the high band input signal HSI and the low band input signal LSI. The output port of the high band low noise amplifier 16H is coupled to the output port of the low band low noise amplifier 16L to achieve the prior art multi-band low noise amplifier 16. Please refer to FIG. 2, which is a functional block diagram of the conventional multi-band low noise amplifier 16 shown in FIG. 1. The multi-band low noise amplifier 16 includes a high band low noise amplifier 16H and a low band low noise amplifier 16L. In each low noise amplifier, a preset bias can be adjusted in a plurality of gain modes according to the received input signal. In any period of time, the multi-band low noise amplifier 16 can operate only in a band mode. For instance, when the received signal is the high RF signal HRF, after being processed by the high band filter 14H, the generated high band input signal HSI will be transmitted to the high band low noise amplifier 16H. In the meantime, the low band low noise amplifier l6L does not operate. The high band low noise amplifier 16H includes a high band receiving port mu InH, three transistors QH1–QH3, adjustable three preset biases BH1–BH3, an internal resistor RBH, and a high band output port OUTH. The high band receiving port InH is used to receive the high band input signal HSI, and the transistors QH1–QH3 can be used to amplify the high band input signal HSI by corresponding gain ratio in various gain modes according to the relative values of the three biases BH1–BH3. At last, an output port OS of the multi-band low noise amplifier 16 can be used to output the amplified high band input signal HSI. When the low band input signal LSI requires being processed, the low band low noise amplifier 16L operates and the high band low noise amplifier 16H does not. Similar to the above-mentioned characteristics of the high band low noise amplifier 16H, the low band low noise amplifier 16L also includes a low band receiving port InL, three transistors QL1–QL3, three adjustable preset biases BL1–BL3, and a low band output port OUTL. The preset biases BL1–BL3 can be arranged so that the low band low noise amplifier 16L can operate in various gain modes. An output port OS of the multi-band low noise amplifier 16 can be used to the processed low band input signal LSI. The output port OS is shared by the high band low noise amplifier 16H and the low band low noise amplifier 16L.
Please note that, first, in the prior art receiver 10, the high band output port OUTH of the high band low noise amplifier 16H is coupled to the low band output port OUTL of the low band low noise amplifier 16H to be integrated as the multi-band low noise amplifier 16 with a single output port (the output port OS). The coupled nodes (OUTH, OUTL) are equivalent to the output port OS of the multi-band low noise amplifier 16. Moreover, when being implemented, the number of band modes is probably more than two (high/low), and the number of low noise amplifiers for processing the band modes increases while increasing the number of band modes; that is, no matter what the number of the low noise amplifiers is, in the prior art, the output ports of the (single-band) low noise amplifiers are coupled to each other so as to be integrated into the multi-band low noise amplifier with a single output. However, the output port of each low noise amplifier is a high impedance node of the low noise amplifier. After the output ports of the low noise amplifiers are coupled to each other, impedance value of the coupled node is also high. Please refer to both FIG. 1 and FIG. 2. As shown in FIG. 2, the impedance value of the high band output port OUTH of the high band low noise amplifier 16H is mainly contributed by an internal impedance ZLH. Similarly, the impedance value of the low band output port OUTL of the low band low noise amplifier 16L is mainly contributed by an internal impedance ZLL. The internal impedances ZLH, ZLL both have high impedance values so the output port OS of the multi-band low noise amplifier 16 shown in FIG. 1 also has a high impedance. Please note that, in the receiver 10 shown in FIG. 1, since the output port OS of the multi-band low noise amplifier 16 is the node at which the output ports of a plurality of (for example, two) low noise amplifiers are coupled to, an effective parasitic capacitor Cp is generated so the high impedance of the output port OS (combined with parasitic capacitor Cp) will lead to a decay of the output signal and a deteriorated frequency response performances of the multi-band low noise amplifier 16.